New construction is simple compared to an addition or remodel project. The insulation values
are usually consistent in a new house, but in an older home, it may vary from room to room.
It is not sufficient to merely supply the additional room(s) with the calculated amount
of heat, as it may throw the existing rooms out of balance. Sometimes it is necessary to
supply the new room with an incorrect amount to keep it proportional to the rest of the
house. In severe situations, the newer area may need to be heated as a separate zone.In
any case, a heat loss/gain should be performed on the existing rooms as well, and the existing
system should be evaluated to determine how well it conforms to the calculated requirements.
A new heat loss will also tell you if the existing heating system can accommodate the additional
load requirements of the addition.

Also
keep in mind that the rooms adjacent to the addition have lost exposure to the outside and
will need less heat.

These same conditions also apply to remodeling projects, where the addition or removal
of windows can seriously affect the room's heating needs as will adding doors or raising
ceilings.

Be pre-warned, if you build a utility room to enclose your
fossil fuel heating system, you must allow for combustible air access. The burners require
specific amounts of air based on their btu output and could create a hazardous condition
without this necessary air. The door to this room must also open into this room. If there
is not enough air into this room and a service man opens the door to inspect the system,
this will supply the necessary air for combustion. If an explosion takes place, the door
will close and be stopped by the door frame rather than open violently into the service
man.